Sulfite oxidase is an enzyme located in the intermembranal space of the mitochondrion. The activity of this enzyme in the rat hepatic mitochondria declines during intoxication by carbon tetrachloride. The decline in activity is coincident with a disruption of the outer membrane of the mitochondrion and a decrease in the efficiency of oxidative phosphorylation. Major aspects of the restoration of mitochondrial function are the return of respiratory control and the restoration of an intact outer membrane. These events precede the return of normal sulfite oxidase activity in the recovering animal. The delayed return of sulfite oxidase activity provides a system for experimental study. The presence of messenger RNA for sulfite oxidase (as revealed by synthesis of immunoprecipitable material in an in vitro reticulocyte translation system), the in vivo incorporation of labelled amino acids into sulfite oxidase, and the synthesis of outer membrane phospholipids during the recovery from acute CCl4 exposure are under study to define the role of these processes in the recovery of the cell following damage induced by a potent hepatotoxin.